The allegations sound like reports of war crimes committed by someone else's soldiers: men in black ski masks enter a house, where three of them take turns raping a 14-year-old girl. They then kill her, her parents, and her 5-year-old sister.

It is the kind of atrocity Americans associate with the Nazis, Serbian paramilitary commandos in Kosovo, perhaps Russian troops in Chechnya -- not U.S. soldiers.

"One doesn't expect the American troops to behave the same way, because there are notions that higher morals prevail in the U.S. armed forces," said Robert Rotberg, an expert on conflict and conflict resolution at Harvard University.

But as a military tribunal in Baghdad is deciding whether five American soldiers must stand trial in connection with the rape and murder of an Iraqi girl and the killing of her parents and sister in March, military experts and historians warn that it will become increasingly difficult for American troops fighting against an elusive enemy in Iraq to maintain military discipline under the intense pressures of war. Wartime atrocities, they say, occur in most wars and are committed by most, if not all, occupying troops -- even by such a high-tech, well-trained military as the United States'.

"Combat is about stress, and criminal behavior toward civilians is a classic combat stress symptom," said John Pike, director of GlobalSecurity.org, a Washington military think tank. "If you get enough soldiers into enough combat, some of them are going to murder civilians."

Recent allegations of atrocities by American troops -- which include the investigations into whether U.S. servicemen shot in cold blood 24 unarmed Iraqi civilians in Haditha in November, shot an unarmed Iraqi man in February, executed a civilian in April and three prisoners in May -- "aren't surprising at all," said Andrew Wiest, professor of military history at the University of Southern Mississippi. "The fact that we maybe weren't expecting them is surprising."

In fact, historical accounts of past wars spell out a grisly pattern of atrocities against civilians. British troops executed and raped civilians during the Revolutionary War, according to U.S. historian David Hackett Fischer's Pulitzer Prize-winning book "Washington Crossing"; Red Army soldiers raped an estimated 100,000 Berlin women between 1945 and 1948, as described by the British historian Antony Beevor's "Berlin: The Downfall 1945." During the Korean War, U.S. commanders repeatedly ordered their troops to kill Korean refugees caught on the battlefield. And last Sunday, the Los Angeles Times published details of a once-secret Pentagon archive that describes 320 alleged incidents of American atrocities against Vietnamese and Cambodian civilians -- not including the 1968 My Lai massacre, in which U.S. troops killed more than 300 Vietnamese civilians in the course of three hours, and which became a turning point in Americans' perception of the Vietnam War.

Since the war in Vietnam, the U.S. military has abandoned the draft, raised its recruiting standards, tightened its rules of conduct in war zones -- outlawing, for example, alcohol consumption or sex during deployments -- and introduced mandatory courses on warrior ethics in Army and Navy colleges.

Even so, "It's difficult to get through to cadets, officers and (enlisted) men the importance of targeting only enemy combatants, taking prisoners and not just shooting anybody," said Mark Grimsley, professor of American military history at the Ohio State University who has spoken at the U.S. Military Academy in West Point and who runs the blog WarHistorian.org. "Some officers are very concerned about these things, and do a good job of training their men. Others are more slipshod about it."

U.S. servicemen allegedly involved in the deaths of Iraqis include a major, three captains, a first lieutenant, and several noncommissioned officers. Soldiers who are alleged to have executed three Iraqi prisoners in May say a U.S. Army colonel had instructed them to kill all fighting-age men.

Since the war began in 2003, at least 14 U.S. servicemen have been convicted in criminal cases stemming from deaths of Iraqis, and at least six other cases, involving 27 servicemen, are pending investigation. Compared with other counterinsurgencies, "there might be fewer (such incidents) than I might expect," said Wiest.

But the longer the U.S. forces remain in the country, the higher the likelihood of new crimes against civilians, warned Raymond Scurfield, a sociologist who served as an Army social worker in Vietnam and who has written about the psychological effects of war on veterans.

"Anybody can maintain discipline for a short period of time," he said. "It's the protracted, repeated stuff that becomes very difficult. As the war is prolonged and becomes nastier, as (American servicemen) are put into very difficult situations, as the civilian populace doesn't come out friendly and aid Americans -- all those dynamics are going to make such incidents happen more frequently."

Winslow Wheeler, an expert at the Center for Defense Information, said atrocities stem from the abusive attitude of most American servicemen toward Iraqis, which was evident from the beginning of the war, when Lt. Peter Katzfrey of the 299 Engineer Battalion, 4th Infantry Division in Tikrit summed up the rules of engagement in an interview with the Chronicle as "shoot to kill. No questions asked."

The difference between "trigger-happy American soldiers who would shoot at vehicles from checkpoints" and the deliberate rape and murder of the Iraqi family "is in degree, but not in nature," Wheeler argued. "Both actions reflect contempt both toward a country and the civilians in it."

The nature of the conflict, in which elusive insurgents in civilian clothes kill Americans and Iraqis by roadside bombs, makes it harder for American troops to discern civilians from enemy combatants. The stress of an incessantly increasing threat amid an escalating sectarian conflict is taking a psychological toll on U.S. forces, some of whom are now completing their third deployment in Iraq in three years.

"It's a very frustrating form of warfare, you always have to be on your guard, you don't know where the attack will come from," said Anthony Dworkin, director of the Crimes of War Project in Washington.

The war's increasing toll on the troops' psyche coincides with the military's need to fill the ranks, which has pushed the Army to lower recruiting standards in autumn that had been set to ensure the quality of the force.

"When you look at the circumstances of whom we send and what we expect them to do, it's surprising we don't have more of those cases," said Loren Thompson, defense analyst at Lexington Institute in Arlington, Va. Former Pfc. Steven Green, who allegedly organized the rape and murder of the Iraqi girl and the killing of her family, was discharged from the Army because he suffered from anti-social personality disorder.

Of about 40,000 soldiers discharged from the Army in 2005, 1,038 were dismissed because of personality disorders, said Lt. Col. Bryan Hilferty, spokesman for the Army's personnel department.

The only way to keep future atrocities to the minimum is by "constant reinforcement that the moral universe still applies in war," said Grimsley.

"If soldiers aren't diligently trained to understand the kinds of frustrations and stresses that tend to generate atrocities and aren't conditioned to avoid them ... the frustrations can take hold and you can wind up going off and doing something like what occurred in Haditha."